Water cooling is the big news for BMW's latest GS. Although the flagship boxer twin still utilizes 'precision' liquid cooling, the system's liquid is now a water/glycol mixture rather than oil. The atmosphere still does most of the work, with the ratio now at 65 percent air-cooling and 35 percent water, adjusted from the previous mode's 78 percent air-cooling and 22 percent oil cooling.
The 101mm bore and 73mm stroke of the famous BMW R1200GS engine is unchanged for 2013, but the valves are 1mm larger on each side and the compression ratio has been raised from 12:1 to 12.5:1.
The characteristic horizontal cylinders are integrated into the construction of the case, rather than bolting on, for the first time in the Boxer's 90-year history. The most noticeable change is that the cylinders are rotated forward to inhale through the top of the head from an airbox directly above the engine. Conversely, the exhaust ports breathe out the bottom of the cylinders, although the pipes still angle forward before looping back to a single collector under the engine. The new BMW ride-by-wire engine management controls 52mm throttle bodies, which was 50mm on the previous model. It's somewhat amazing that BMW is capable to increase the R1200GS performance with 15 horsepower to a claimed 125 horsepower at 7700rpm, with torque posted as 125Nm at 6500rpm.
BMW has selected a wet, multi-plate slipper clutch, which represents another departure from traditional BMW R-class engineering, while the gearbox being integrated in the case is a first for the boxer engine layout. The new clutch pack is mounted in the front of the case where it gets power directly from a primary gear on the front of the crankshaft. The output shaft runs parallel to the crank but offset to the right where it delivers power past the big-ends to the new gear set mounted aft. Encircling the clutch output shaft is the counterbalancer shaft, which runs at engine speed, powered by another gear mounted on the rear of the crank. The clutch plates are actuated hydraulically, while a mechanical ramped-slipper mechanism – much like one seen in sportsbikes for nearly a decade – mitigates chatter under deceleration.
The new BMW R1200GS engine is a modern marvel of German engineering.